With the rapid development of digitalization of information, digitalization in image processing is increasingly required. In digital cameras in particular, solid-state image pickup devices, such as Charge Coupled Devices (CCD) and Complementary Metal Oxide Semiconductor (CMOS) sensors, have been mainly provided on imaging planes instead of films.
In imaging devices including CCDs or CMOS sensors, an image of an object is optically taken by an optical system and is extracted by an image pickup device in a form of an electric signal. Such apparatuses may be used in digital still cameras, video cameras, digital video units, personal computers, mobile phones, PDAs, image inspection apparatuses, industrial cameras used for automatic control, and the like.
An imaging device, in which light is regularly dispersed by a phase plate and is reconstructed by digital processing to achieve a large depth of field, has been suggested. In the present market for imaging devices, miniaturization is desired. In order to meet this need for miniaturization, a lower height of a lens unit and a higher sensitivity for imaging devices is desired.
If a micro lens array is arranged at the pre-stage of a solid-state imaging element in order to improve the sensitivity, the quantity of light that enters each pixel of the solid-state imaging element can be efficiently collected. The light collecting efficiency to the pixel of the solid-state imaging element improves if the micro lens corresponding to the pixel of each solid-state imaging element is used. However, if the center of the micro lens is arranged on the normal line of the center of the pixel of each solid-state imaging element, the light collecting efficiency lowers as the light diagonally enters at the lens arranged at the peripheral part. As a result, shading in which the peripheral part becomes a dark image occurs. Consideration is made in shifting the position of the micro lens towards the center side in the direction of the peripheral part, but this is not sufficient. Thus, further increasing the quantity of light that enters each pixel and enhancing the sensitivity are desired.
The shading that occurs caused by various complex factors can be appropriately corrected by combining a plurality of correcting functions for every factor. However, reducing the shading by a simple method is desired as the processing amount of the correction by image signal processing significantly increases with increase in the number of pixels and the condition parameters.
In addition, in the known imaging devices, it is premised that a Point Spread Function (PSF) obtained when the above-described phase plate is placed in an optical system is constant. If the PSF varies, it becomes difficult to obtain an image with a large depth of field by convolution using a kernel.
Therefore, setting single focus lens systems aside, in lens systems such as zoom systems and autofocus (AF) systems, there is a problem in adopting the above-mentioned structure because high precision is required in the optical design and costs are increased accordingly. More specifically, in known imaging devices, a suitable convolution operation cannot be performed and the optical system must be designed so as to eliminate aberrations, such as astigmatism, coma aberration, and zoom chromatic aberration that cause a displacement of a spot image at wide angle and telephoto positions. However, to eliminate the aberrations, the complexity of the optical design is increased and the number of design steps, costs, and the lens size are increased.
Since a phase plate may set OTF almost constant to the broad object distance, a point image can have a rotationally asymmetric shape. As a result, using an optical system with an image processing, a picture with a large depth of field can be taken. However, the constant OTF may cause a degradation of contrast. If an image processing such as a convolution process is used in order to improve such degraded contrast, a noise would increase as a result.
Therefore, there is a need for simplified image processing systems, specifically, an optical system which can achieve a high contrast without a large noise.